Seawater and Salinity

• at T=4°C -> lowest vibration before hexagons start to form -> highest density

• at T>or< 4°C -> abundance of hexagons increases/vibration increases -> density increases

• at T<=°C -> all H2O held in hexagons (ice crystal lettice structure) -> all H2O molecules farthest apart --> lowest possible density .> ice 8% larger -> floats

• fresh and warm waters are the lightest whilst cold and saline waters are the heaviest

• deep water formation occurs when warm surfce water cools down, e.g. in the North Atlantic

• driver of the ocean conveyor belt

Generally: warm and fresher water floats (lower density) and cold and saline water sinks (higher density). Deep water formation occurs at high latitudes e.g.: NADW/AAIW/AABW, when it cools down during its journey to colder polar waters. Additionally, the formation of ice, and the rejection of brine, might further increase the waters density, causing it to “drop” and form deep-waters. The thereby created “vacuum” is replenished with warmer and less saline surface waters, resulting in a global ocean circulation. The overall driver of this is the differing radiation budget on a global scale (e.g.: equator higher solar energy input, than in polar regions –> affects temperature).

Cations:

• Na+

• Ca2+

• K+

• Mg2+

Anions:

• Cl-

• HCO3-

• Br-

• SO4-

Potassium, as Ca is involved in CaCO3-formation

Remember residence times: τ = mean conc. in ocean * (total ocean vol. / inflow per yr)

Although inflow per year of Ca2+ is higher (major cation of river discharge), mean concentration of Ca2+ in ocean is way lower (used in calcification→CaCO3)→K+ has the higher residence time, it is not subject to biological depletion, it behaves conservative.

Conservatice = long residence times (RT>>>ocean mixing)

• Uptake

• Precipitation

• Scavenging

• Additional supply by sediments

• Aerosols

• Rivers

molality = mol/kg H2O

molinity = mol/kg SW

Molinity incorporates the density differences occurring for seawater of different salinity.

Molinity (mol/kgSW)

-> calculation with SW have less error

-> conservative with changes in pressure and temperature

-> time that a molecule spends in the ocean

-> defined by reservoir and flux (in and out)

-> time of one turnover

influx = outflux

reservoir = constant

Iron: 200-500 yrs

Phosphate: 69.000 yrs

-> scavanginng leads to iron fallout

-> P is repeatedly reused

PO43- has the longer residence time in the ocean. Phosphate, although being an essential component for building up biomass is almost never limiting. However, iron, as an important co-factor for many enzymatic processes is quickly taken up and stripped out of the seawater, once it is dissolved. Furthermore, iron is quickly precipitated under oxic conditions.

Br-: 100 Mio. yrs

H2O: 40.000 yrs

-> freezing: dissolved salts function as a structure breaker, no H-bonds, less hexagons

-> circulation: saline water is heavier than fresh water

-> solubility of gases: saline water has less dissolved gases

-> river input

-> mineral input (weathering)

-> evaporation

-> precipitation (direct)

-> groundwater flow

-> ice formation and brine rejection

-> ice melting

-> precipitation

-> biological activity

-> external input

At 3.98°C.

Vibration energy controls distance between molecules. At 3.98°C the minimal distance is reached and the molecules start forming hexagonal structures which reduces the density. Salt is a structure breaker and allows higher density at lower temperatures as well as lower freezing point.